There have been disclosed a technique for forming a transparent conductive film from a composition containing indium nitrate (see Patent Document 1), and a technique for forming a metal oxide semiconductor layer from a composition containing indium nitrate (see Patent Document 2).
Such a transparent conductive film and a metal oxide semiconductor layer have a conducting property. However, an important aspect of the conducting property required for the metal oxide semiconductor layer is mobility, and that for the transparent conductive film is conductivity.
Conductivity is defined as a product of mobility and carrier concentration. Thus, when the carrier concentration is high, high conductivity can be attained even if the mobility is low. Also, in the case where difficulty is encountered in enhancing conductivity, satisfactory current can be obtained when the thickness of the transparent conductive film increases. The effect is equivalent to enhancement in conductivity. Under such circumstances, efforts are made on improvement mainly focusing on formation of a thick transparent conductive film.
On the other hand, layer thickness is not important for a metal oxide semiconductor layer. Actually, when the metal oxide semiconductor layer has a thickness of about 10 nm, satisfactory performance can be attained. In contrast, when the layer thickness is excessively large, mobility may be impaired due to cracking of the layer or the like. Thus, it is desired to attain high mobility by a thin film having a thickness of about 10 nm. Since the metal oxide semiconductor layer exhibits excellent performance when the thickness is small, surface roughness, which is not problematic for transparent conductive film, may considerably affect the semiconductor characteristics. This is a main problem to be solved in the case of a metal oxide semiconductor.
More specifically, in the case where a high-performance metal oxide semiconductor layer is formed through a coating technique, satisfactory performance of a metal oxide semiconductor layer fails to be attained, if a conventional transparent conductive film forming composition is used without any improvement.
Patent Document 1 discloses a transparent conductive film formed from indium nitrate and a carbitol. However, mobility and roughness of the film are not evaluated, and the document is silent to application of the transparent conductive film to a metal oxide semiconductor layer. In addition, the difference in characteristics between ethylene glycol and a carbitol is not described.
Patent Document 2 discloses a metal oxide semiconductor formed of indium nitrate and ethyl carbitol. In Patent Document 2, ethylene glycol monomethyl ether and propylene glycol monomethyl ether are described as particularly preferred solvents. A liquid mixture containing such a solvent can attain excellent semiconductor characteristics. However, the present inventor carried out a reproducing test and, as a result, a film obtained from the metal oxide semiconductor was found to be readily provided with pinholes, and to have considerable roughness, which are problematic. Furthermore, in Patent Document 2, the coated substrate is heat-treated (dried) at relatively high temperature (100° C.). From an industrial viewpoint, drying the substrate at high temperature is not practical, and drying at a temperature lower than 100° C. is preferred. However, drying at lower temperature may result in a rise in surface roughness.
As described above, a low-boiling-point solvent such as propylene glycol monomethyl ether is not a suitable solvent, in that the solvent causes to generation of pinholes in the film. A high-boiling-point solvent such as ethylene glycol provides improved film-formability, but readily remains in the film. As a result, difficulty is encountered in forming a metal oxide semiconductor film having excellent mobility. Thus, there has never been developed a metal oxide semiconductor layer forming composition which can provide a flat metal oxide semiconductor film with high mobility and no pinhole.